Solar-like oscillations in a metal-poor globular cluster with the HST

We present analyses of variability in the red giant stars in the metal-poor globular cluster NGC6397, based on data obtained with the Hubble Space Telescope. We use an non-standard data reduction approach to turn a 23-day observing run originally aimed at imaging the white dwarf population, into time-series photometry of the cluster's highly saturated red giant stars. With this technique we obtain noise levels in the final power spectra down to 50 parts per million, which allows us to search for low amplitude solar-like oscillations. We compare the observed excess power seen in the power spectra with estimates of the typical frequency range, frequency spacing and amplitude from scaling the solar oscillations. We see evidence that the detected variability is consistent with solar-like oscillations in at least one and perhaps up to four stars. With metallicities two orders of magnitude lower than of the Sun, these stars present so far the best evidence of solar-like oscillations in such a low metallicity environment.Comment: 7 pages, 6 figures, accepted by Ap

COST Action ES0901: European procedures for flood frequency estimation (FloodFreq) [Keynote]

The aim of COST Action ES0901 European Procedures for Flood Frequency Estimation (FloodFreq) is to undertake a Pan-European comparison and evaluation of different methods for flood frequency estimation under the various climatologic and geographic conditions found in Europe, and different levels of data availability. A scientific framework for assessing the ability of these methods to predict the impact of environmental change (climate change, land-use and river engineering works) on future flood frequency characteristics (flood occurrence and magnitude) will be developed and tested. The availability of such procedures is crucial for the formulation of robust flood risk management strategies as required by the Directive of the European Parliament on the assessment and management of floods. The outputs from FloodFreq will be disseminated to: academics, professionals involved in operational flood risk management from private and public institutions, and relevant policy makers from national and international regulatory bodies. This Action enable cooperation between researchers involved in nationally funded research projects to, thereby enabling testing of methods free from the constraints of administrative boundaries, and allowing a more efficient use of European flood research funding

Correcting stellar oscillation frequencies for near-surface effects

In helioseismology, there is a well-known offset between observed and computed oscillation frequencies. This offset is known to arise from improper modeling of the near-surface layers of the Sun, and a similar effect must occur for models of other stars. Such an effect impedes progress in asteroseismology, which involves comparing observed oscillation frequencies with those calculated from theoretical models. Here, we use data for the Sun to derive an empirical correction for the near-surface offset, which we then apply three other stars (alpha Cen A, alpha Cen B and beta Hyi). The method appears to give good results, in particular providing an accurate estimate of the mean density of each star.Comment: accepted for publication in ApJ Letter

P-mode observations on Alpha Cen A

We have made a clear detection of p-mode oscillations in the nearest solar-like G2V star $\alpha$ Cen A with the CORALIE spectrograph on the 1.2-m Swiss telescope at the ESO La Silla Observatory. We report the 5 nights of observation on this star in May 2001 during which 1260 high precision radial velocity measurements were obtained. The power spectrum clearly shows several identifiable peaks between 1.7 and 3 mHz. The average large splitting of 105.7 $\mu$Hz and the amplitude of about 35 cm s$^{-1}$ of these modes are in agreement with theoretical expectations.Comment: 4 pages, 4 figures, A&A Letter accepte

Joint Time Frequency Analysis Techniques: A study of Transitional Dyamics in Sheet/Cloud Cavitation

An introduction to, and the use of, Joint Time Frequency Analysis techniques is given. Special emphasis is made on Time Frequency Distribution series. This method is demonstrated on selected experimental data. Attention is given the transition region in the dynamic nature of sheet/cloud cavitation. Wavelets and a method that is a variation of phase-portraits are also presented and discussed with regard to cavitation dynamics. An example on how experimental techniques have been improved by the above mentioned methods is given

Scaled oscillation frequencies and echelle diagrams as a tool for comparative asteroseismology

We describe a method for comparing the frequency spectra of oscillating stars. We focus on solar-like oscillations, in which mode frequencies generally follow a regular pattern. On the basis that oscillation frequencies of similar stars scale homologously, we show how to display two stars on a single echelle diagram. The result can be used to infer the ratio of their mean densities very precisely, without reference to theoretical models. In addition, data from the star with the better signal-to-noise ratio can be used to confirm weaker modes and reject sidelobes in data from the second star. Finally, we show that scaled echelle diagrams provide a solution to the problem of ridge identification in F-type stars, such as those observed by the CoRoT space mission.Comment: accepted for publication in Communications in Asteroseismolog